(1) Field of the Invention
This invention relates to apparatus for use in detecting the level of a selected analyte in a biological fluid. More particularly, the invention relates to a method and apparatus for effecting an electrical connection between an electrometer and an analysis slide in a potentiometric analyzer.
(2) State of the Prior Art
Various types of probes are known in the prior art for forming an electrical connection with a relatively thin layer of conductive material deposited on a substrate. For example, in the field of microelectronics, small, accurately-positioned probes have been developed for testing components of a minute size. In the patent to Kvaternik, U.S. Pat. No. 4,035,723, there is disclosed a preloaded probe which is flexed, upon contact of the probe with the surface of a circuit pad, to achieve a bidirectional movement, or scrubbing, of the probe on the surface. The scrubbing action is necessary in the event the surface of the circuit pad is oxidized; however, only a limited amount of scrubbing is desirable, since electrical leads are bonded to the surface after the test.
The patent to Oates, U.S. Pat. No. 3,453,545, discloses apparatus for sequentially testing a plurality of devices on a semiconductor wafer. A mechanism is provided for vibrating the wafer during engagement of a probe mechanism to effect a good connection between the mechanism and the wafer.
The patents discussed above are directed to solutions to the problem of making an electrical connection with a thin conductive layer which is formed on a relatively hard substrate such as a ceramic material. Further, these patents are concerned with forming an electrical connection on the conductive layer, without damaging the layer surface for subsequent electrical use. The Oates patent teaches the undesirability of tearing or gouging through the oxide layer on the contacts because it tends to damage the thin metallic contacts, and further, because it produces nonuniform and nonreproducible test results.
Recent developments have provided analysis slides which can be used to potentiometrically designate the activity of the ions in a liquid test solution. Such slides are formed with a pair of ion-selective electrodes which include thin conductive layers deposited on a relatively soft backing material such as a plastic film. In certain of these slides, a silver chloride layer is formed over the conductive layer; in other configurations, a nickel layer is formed over the conductive layer. A good electrical connection is essential in these slides, since the signal produced in the slide is relatively weak; in some cases, the signal is less than one millivolt. In forming an electrical connection with the analysis slide, a probe must pierce through any layers which overlie the conductive layer. However, the relatively soft backing material creates special problems in forming an electrical connection with the conductive material, since the backing material tends to distort when a probe unit is brought into contact with the layers; thus, the probes do not break through to the conductive layer.
One attempt to overcome the problem of forming an electrical connection with the above-described analysis slide is described in "Research Disclosure," Vol. 175, November 1978, Publication No. 17569, published by Industrial Opportunities Limited, Homewell, Havant Hampshire, PO9 1EF, United Kingdom. The disclosed device comprises a probe biased against the slide by a spring having a relatively-high spring constant. It has been found, however, that simply increasing the spring force on the probe was not totally successful in forming a good electrical connection, since, as noted above, the backing material tended to distort such that the probe was not getting through to the conductive layer.